This invention relates to apparatus for determining the length of a moving elongate element. The invention is particularly, but not exclusively suitable for measuring the length of electric cables, conductors etc., but the generic term elongate element is herein meant to include flat sheet like items such as paper or metal.
Known techniques of length measurement for moving items can be defined as either contact or non-contact techniques. Contact methods consist of providing some moveable member, usually a wheel or other roller, which contacts the element as it moves. Movement of the elongate element causes corresponding movement of the roller which can be detected to give an indication of the length of the element. During movement of the element however, it is common for there to be slippage between the contacting roller and the element, thereby giving a false indication of its length. Attempts to prevent slippage in contact systems have never been entirely successful.
Non-contact methods such as laser anemometry avoid the problem of slippage but are relatively expensive. It is an object of the present invention to provide a contact technique which is less prone to false measurement due to slippage.